The present invention relates to a method of mounting a semiconductor device whereby the semiconductor device is connected with, and fixed to the surface of a circuit board with the use of an anisotropic conductive adhesive.
A method of mounting a semiconductor device whereby the semiconductor device is connected with, and fixed to the surface of a circuit board with the use of an anisotropic conductive adhesive has been put to practical use. The conventional method of mounting a semiconductor device is described hereinafter with reference to FIGS. 11 to 16.
FIGS. 11 to 14 are sectional views showing respective steps of the conventional method of mounting the semiconductor device. As shown in these figures, with the conventional method of mounting the semiconductor device, wiring patterns 15 provided on a circuit board 17 are connected with bumps 14 formed on a semiconductor device 16, respectively. When making such a connection, conductive particles 12 having electric conductivity, contained in an anisotropic conductive adhesive 13, are sandwiched between the respective wiring patterns 15 and the respective bumps 14, thereby effecting electric conduction therebetween by the agency of the conductive particles 12.
The anisotropic conductive adhesive 13 is composed of the conductive particles 12 which are mixed in an adhesive resin 11 of a thermosetting type, made of an epoxy based adhesive, so as to have electric conductivity, and is formed in a film-like shape on a base film while being protected by a cover film. The conductive particles 12 are metallic particles made of silver, solder, or so forth, 5 to 10 xcexcm in diameter, or composed of resin particles made of a plastic, with gold (Au) plating applied on the surface thereof.
The circuit board 17 is provided with the wiring patterns 15 formed on a substrate thereof, made of glass epoxy resin, ceramic, or glass. The wiring patterns 15 are formed of copper or gold, or composed of an indium tin oxide (ITO) film, or so forth for use in a liquid crystal display panel, and the like.
An operation of mounting a semiconductor device comprises the following steps.
First, as shown in FIG. 11, the anisotropic conductive adhesive 13 is transferred onto a part of the circuit board 17 where the semiconductor device 16 is to be connected.
In a next step, as shown in FIG. 12, the semiconductor device 16 is placed on the circuit board 17 disposed opposite thereto, after aligning the respective wiring patterns 15 provided on the circuit board 17 with the respective bumps 14 formed on the semiconductor device 16.
Subsequently, the semiconductor device 16 is thermally press-bonded to the circuit board 17 by heating the semiconductor device 16 while applying a pressure P thereto by use of a heating and pressurizing tool 18 provided with a built-in heater as shown in FIG. 13, thereby curing the adhesive resin 11 contained in the anisotropic conductive adhesive 13.
Then, as shown in FIG. 14, upon curing of the adhesive resin 11, the semiconductor device 16 is bonded to, and fixed to the surface of the circuit board 17, so that electric conduction is maintained between the respective bumps 14 of the semiconductor device 16, and the respective wiring patterns 15 through the intermediary of the conductive particles 12 contained in the anisotropic conductive adhesive 13.
With this method of mounting the semiconductor device, a thermal press-bonding process proceeds from the outset at a temperature required for curing the adhesive resin 11 by use of the heating and pressurizing tool 18, however, the adhesive resin 11 of the thermosetting type, such as an epoxy based adhesive, and so forth, softens up rapidly when heated to more than a certain temperature, and is turned into a half-fusion state. Thereafter, a curing reaction proceeds, thereby curing the adhesive resin 11. FIGS. 15 and 16 are enlarged views showing conditions of a structure for mounting the semiconductor device before, and after the thermal press-bonding process, respectively.
FIG. 15 shows the condition when the semiconductor device 16 is placed on the circuit board 17, that is, the condition before heating and pressurizing are carried out by use of the heating and pressurizing tool 18 as shown in FIG. 13, showing an enlarged view of a part of the semiconductor device 16, and the circuit board 17, respectively, so as to illustrate with clarity the state of the conductive. particles 12 existing between the bumps 14 and the wiring patterns 15. At this point in time, preceding the heating and pressurizing, the adhesive resin 11 remains in the film-like shape as transferred.
Subsequently, as shown in FIG. 16, when a pressure P is applied to the semiconductor device 16 while applying heat thereto a temperature at which curing reaction occurs to the adhesive resin 11 by use of the heating and pressurizing tool 18 in order to cause the adhesive resin 11 to be cured, the adhesive resin 11 is cured rapidly, and at the same time, is crushed and fluidized, thereby being forced out of under the semiconductor device 16. Since a spacing between the respective bumps 14 and the respective wiring patterns 15 is narrowed down to a minimum, portions of the adhesive resin 11, in the vicinity of the respective bumps 14, is forcefully pushed out by the respective bumps 14, and are fluidized at the maximum.
Along with the adhesive resin 11, which has been fluidized, the conductive particles 12 are also pushed out outside under the respective bumps 14, and consequently, when the thermal press-bonding process is completed upon curing of the adhesive resin 11 later on, many of the conductive particles 12 will be found to be present in regions lying off the bottom face of the respective bumps 14. Accordingly, there will remain only a few of the conductive particles 12 between the respective bumps 14 and the respective wiring patterns 15.
Thus, a problem has been encountered with the conventional method of mounting the semiconductor device in that connecting resistance between the respective bumps 14 and the respective wiring patterns 15 became high because of a temporary softening phenomenon occurring to the adhesive resin 11 of the anisotropic conductive adhesive 13 prior to curing thereof caused by heating during the thermal press-bonding process, and pushing out of the conductive particles 12 and a decrease in the number of the conductive particles 12 left out between the respective bumps 14 and the respective wiring patterns 15 caused by pressurizing force applied to the semiconductor device.
In order to hold down the connecting resistance at a low level, it is required that as many as possible of the conductive particles 12 are left out between the respective bumps 14 and the respective wiring patterns 15. To this end, it is necessary to expand an area of a connecting region where both the parts described are to be connected with each other so that many of the conductive particles 12 can be arrested therebetween.
However, in order to implement high-density mounting, it is necessary to reduce an area of the connecting region, thereby rendering it difficult to arrest many of the conductive particles 12 between the respective bumps 14 and the respective wiring patterns 15. Consequently, the number of the conductive particles 12 left out in the connecting region will become fewer. Accordingly, the conventional method of mounting the semiconductor device described in the foregoing is unsuitable for high-density mounting because a connecting resistance value becomes high, and connection tends to become unstable.
The invention has been developed to solve such a problem as described above with the conventional method of mounting the semiconductor device using the anisotropic conductive adhesive, and it is therefore an object of the invention to enable stable connection to be effected at low connecting resistance even if the area of the connecting region is small by increasing the number of conductive particles arrested between the respective bumps of the semiconductor device and the respective wiring patterns of the circuit board, thereby rendering the method suitable for high-density mounting.
To this end, the method of mounting a semiconductor device using an anisotropic conductive adhesive, according to the invention, comprises the following steps (1) to (4).
(1) a step of disposing the anisotropic conductive adhesive, composed of conductive particles mixed in an adhesive resin of a thermosetting type, in a region on the surface of a circuit board, where wiring patterns are formed;
(2) a step of placing the semiconductor device on the surface of the circuit board after aligning bumps provided on the semiconductor device with the wiring patterns formed on the surface of the circuit board, respectively;
(3) a step of provisionally press-bonding the semiconductor device onto the circuit board by applying a predetermined pressure thereto at a temperature lower than the curing temperature of the adhesive resin of the anisotropic conductive adhesive; and
(4) a step of bonding the semiconductor device, as provisionally press-bonded, onto the circuit board by curing the adhesive resin at a temperature causing the adhesive resin to be cured.
With the method of mounting the semiconductor device as described above, electric conduction is preferably effected between the bumps of the semiconductor device and the wiring patterns of the circuit board, respectively, in the step of provisionally press-bonding the semiconductor device onto the circuit board.
Further, the step described under (4) may be performed by means of thermal press-bonding using a heating and pressurizing tool set at a temperature causing the adhesive resin to be cured.
Otherwise, the step described under (4) may be performed by placing the circuit board with the semiconductor device provisionally press-bonded thereto in a furnace set at a temperature causing the adhesive resin to be cured, or by placing the circuit board with the semiconductor device provisionally press-bonded thereto on a hot plate set at the temperature causing the adhesive resin to be cured.
Further, with the method of mounting the semiconductor device, the conductive particles are preferably brought into contact with the bumps of the semiconductor device and the wiring patterns of the circuit board, respectively.
With the method of mounting the semiconductor device according to the invention, since provisional press-bonding is effected at a temperature at which the adhesive resin of the anisotropic conductive adhesive remains in a condition of low fluidity in mounting the semiconductor device on the circuit board, the conductive particles are held sandwiched between the respective bumps and the respective wiring patterns while the adhesive resin is in a condition of high viscosity, so that a large number of the conductive particles are left out therebetween. As final curing of the adhesive resin is effected thereafter, it is possible to achieve a stable connection at a low connecting resistance with many of the conductive particles remaining in the connecting region. Furthermore, since many of the conductive particles can be left out in the connecting region, it is possible to reduce the area of the connecting region, thereby enabling connection to be realized at a still finer connection pitch than before.